you are talking about the effect, not the cause.

Sites are slow because with all that excess performance they do more tracking, instead of keeping the website as-is.

Because that's the only thing business managers can come up with in that situation. It's a misaligned goal of users vs businesses.

In my opinion the "user agent" behavior of browsers is long gone, because they're not acting on behalf of their users anymore. They're acting on behalf of businesses.